1. Field of the Invention
The present invention relates to the field of fiber optics, and more specifically, to the angle fusion splicing of low-temperature non-silica fibers with silica glass fibers.
2. Description of the Related Art
In the field of fiber optics, joining or splicing of optical fibers is a well-known and widely practiced technique. The most common method for splicing of two standard fused silica fibers is based on the fusion of the adjacent ends of the optical fibers that are to be joined. The fibers are brought close to each other and are aligned so that their cores are coaxial with each other. Heat is transferred to both fiber ends by a filament around the fiber or an electric arc between two electrodes that are positioned on either side of the axis of the two optical fibers. This heat is sufficient to soften the glass at the end of each of the two fibers to be joined. The optical fibers are then brought in contact and the hardening of the softened glass occurs as the temperature is lowered below the softening and glass transition temperatures to form a permanent bond between the fibers. See, for instance, D. L. Bisbee, “Splicing Silica Fibers with an Electric Arc”, Applied Optics, Vol. 15, No. 3, March 1976, pp. 796-798. These techniques have been designed for and used to fuse fibers that have the same or very similar material compositions, e.g. two standard silica telecom fibers, in many applications including erbium doped fiber amplifiers (EDFAs).
In many applications, two fibers having different glass compositions and substantially different softening temperatures must be fusion spliced. Typically, a special fiber of some sort is being fusion spliced to a standard silica telecom fiber. The standard fusion splicing process must be modified to accommodate the difference in softening temperatures and provide a low loss (<0.3 dB), low back reflection (<−50 dB) and mechanically reliable fusion splice.
The Asahi Glass Company (AGC) conducted and published an extensive study “Technical Bulleting: Bismuth based EDF—A Broadband, High Efficiency and Compact EDF” on the effectiveness of different glasses to provide compact EDFAs and concluded that Bismuth based glass provided the best overall properties. A key factor in this determination was AGC's ability to form mechanically reliable low-loss fusion splices between Bismuth Oxide fibers and silica fibers and their inability to form such splices with Tellurite, Fluoride and Phosphate glasses, which have lower glass transition and softening temperatures than Bismuth based glass. AGC used an arc discharge at the gap between the fibers to form the fusion splice. However, Bismuth based glass does not provide the gain per unit length or other spectroscopic properties of Phosphate glass.
U.S. patent application Publication Ser. No. US 2001/0047668 A1 published on Dec. 6, 2001 details Asahi's method of fusion splicing Bismuth based on glass fibers with standard silica fibers. The quartz type glass fiber (SF) and the bismuth Bi2O3 based glass fiber (BF) are aligned so that their entire end surfaces are in contact with each other. Then the SF and BF fibers are set so that the arc discharge elements intersect the SF fiber at least 1 micron away from the surface contact. A voltage is applied between the electrodes to heat the glass fibers so that the temperature is highest on the SF fiber at least 1 micron from the surface contact. If the distance is less than 1 micron, significant plastic flow or fugacity may take place at the end surface of the BF fiber, whereby the abutted end surface can not appropriately be fusion-spliced, and connection loss tends to be significant.
Asahi's publication also provides for forming an angle-splice, i.e. where the angle formed by the end surfaces with respect to the fiber axis is less than 90°, to reduce reflection of light off the splice due to the difference in refractive index between the SF and BF fibers. Cleaving both fibers with complementary angles between 60° and 87°, carefully aligning the fibers so that their entire end surfaces are in contact with each other and then fusing the fibers as described above, forms the angle splice. Aligning two complementary angle-cleaved fibers is a much more difficult and time-consuming task than aligning a pair of square end fibers. Hence, the yield and cost are more expensive. Furthermore, angle cleaving specialty fibers such as Bismuth based fiber is a difficult low yield process because the mechanical strength of these glasses is weaker than silica glasses.
There remains an industry need for a low-cost high-yield method of angle fusion splicing low-temperature multi-component glasses such as Phosphate, Bismuthate and Tellurite to standard silica fibers for use in compact EDFA and other telecom applications.